1. Field of the Invention
The present invention generally relates to a method of providing unequal error protection in a multi-carrier transmission, and relates to a coding device and a decoding device for performing such a method. The present invention particularly relates to a method of providing unequal error protection by providing different channel qualities for different media in multimedia transmission using a plurality of carriers so as to achieve efficient transmission, and relates to a coding device and a decoding device for performing such a method.
2. Description of the Related Art
In broadband wireless communication, multi-path frequency selective fading is a major factor to deteriorate channel quality, and multi-carrier transmission is a well known scheme to cope with this problem.
FIGS. 19A and 19B are illustrative drawings for explaining the multi-carrier transmission.
As shown in FIG. 19A, the multi-carrier transmission divides a transmission band into a plurality of carrier frequencies f1 through fN (hereinafter called sub-carriers), and each carrier frequency is modulated to transmit data. This produces a frequency diversity effect, which compensates for degradation of transmission quality caused by frequency selective fading, thereby achieving a high-quality wireless communication.
When sub-carriers f1 through fN are orthogonal to each other, each modulated signal can be extracted on the receiver side without any degradation even if carrier spectrums overlap with each other as shown in FIG. 19B. This makes it possible to make use of a narrower frequency band. Such a scheme is called orthogonal frequency division multiplex (OFDM), and is one of the variations of the multi-carrier transmission.
One of the problems of the multi-carrier transmission is an increase in a peak envelope power of a transmission signal or an increase in a ratio of the peak to an average power. In a transmission system having a large peak power, a linear amplifier having a broad range of amplifying levels is necessary in order to maintain linearity of the system. Such a broad-range linear amplifier is generally expensive, and is not attractive in terms of power efficiency. When an inexpensive narrow-range linear amplifier is used, a non-linearity distortion is generated in a saturation region, resulting in a degradation of system performance. These problems impeded an effort to use the multi-carrier transmission in practice.
Measures to suppress the peak power mainly fall into either one of two different categories. One is to place a restriction on input-signal patterns used in the multi-carrier transmission, and the other is to limit the output level of modulated signals of the multi-carrier transmission.
The former measure eliminates an input signal pattern having large peak power due to coding. This insures that the peak power of a transmission signal stays below a certain threshold level. No degradation in performance occurs when this measure is taken. Further, when the minimum code distance is larger than the minimum distance between a signal, a bit error rate can be reduced.
The later measure utilizes the fact that there is only a small probability of having a signal pattern with such a large peak power as to generate a non-linear distortion. By utilizing this fact, when peak power exceeding a predetermined threshold is detected, an excess portion of the peak power is cut off or clipped off, for example. This measure inevitably result in an increase of a side-robe level due to the non-linear distortion. That is, inter-carrier interference is generated. This measure thus entails a degradation in performance.
Another method is to normalize a signal to a threshold level by lowering the level of an entire envelope of the transmission signal when peak power exceeding the threshold value is detected. This method results in a decrease in the signal-to-noise ratio of the received signal, thereby degrading the performance.
In consideration of these, the former measure is preferable for the purpose of achieving broadband high-quality wireless transmission. In high-speed broadband wireless communication, further, multi-media mobile access needs to be achieved so as to handle various media such as image data and audio data.
In multi-media communication, each data type requires different channel quality. In general, channel quality depends on an employed coding method. However, audio transmission requires a bit error rate in the order of 10xe2x88x922, for example, while the transmission of image data requires a bit error rate lower than 10xe2x88x925.
In packet communication, further, control data for the packets requires a higher quality than is required for the data information that contains the multi-media contents. This is because delivery of packets may fail if the control data contains error, and such a failure may cause an unexpected increase in the traffic load on the entire network. In order to prevent this, the control information requires a significantly lower bit error rate.
In order to meet these requirements, a plurality of coding methods having different levels of protection against bit errors may be employed so as to provide different channel qualities, thereby achieving efficient transmission. An unequal error protection method is known to be effective for this purpose.
The unequal error protection method, however, is not directed to the multi-carrier transmission. In other words, there is no unequal error protection method known to date that is directed to a coding process treating a set of multi-carrier symbols as one code word.
Accordingly, there is a need for an unequal error protection method which can provide different channel qualities for different data types in the multi-carrier transmission so as to improve transmission efficiency, and, also, can reduce peak power in the multi-carrier transmission.
Accordingly, it is a general object of the present invention to provide an unequal error protection method which can satisfy the need described above.
It is another and more specific object of the present invention to provide an unequal error protection method which can provide different channel qualities for different data types in the multi-carrier transmission so as to improve transmission efficiency, and, also, can reduce peak power in the multi-carrier transmission.
In order to achieve the above objects according to the present invention, a method of providing unequal error protection in a multi-carrier transmission includes the steps of coding data by a coding process of a given minimum code distance as measured in block codes comprised of modulated-signal points of sub-carriers, and changing the minimum code distance for each multi-carrier-symbol duration by changing the coding process.
According to one aspect of the present invention, the method as described above has the coding process thereof using a set of such codes that multi-carrier-modulated signal has a peak power lower than a predetermined threshold.
It is yet another object of the present invention to provide an encoder and decoder which can implement the unequal error protection method of the present invention.
In order to achieve the object described above according to the present invention, a coding device for providing unequal error protection in a multi-carrier transmission includes a plurality of coding units, each of which codes a corresponding data frame of an information bit signal by a coding process of a given minimum code distance specific to a corresponding coding unit so as to output a coded bit signal, a selector unit which successively selects one of said plurality of coding units, and outputs the coded bit signal supplied from the selected one of said plurality of coding units, and a mapping unit which maps the coded bit signal output from said selector unit to complex signals corresponding to the sub-carriers.
Further, a decoding device for decoding received signals corresponding to a plurality of sub-carriers transmitted in a multi-carrier-transmission system includes a timing control unit which generates data control signals such that each of the data control signals varies over time to present various patterns within a corresponding multi-carrier-symbol duration, a plurality of coding units, each of which codes a corresponding one of the data control signals by a coding process of a given minimum code distance specific to a corresponding coding unit so as to output a coded bit signal, a mapping unit which maps the coded bit signal output from a selected one of said plurality of coding units to complex signals corresponding to the sub-carriers, and a code-distance-calculation unit which calculates a code distance between the complex signals and the received signals with respect to each of the various patterns of the corresponding one of the data control signals within a corresponding multi-carrier-symbol duration, wherein one of the various patterns providing a smallest code distance is selected as a decoding result of the corresponding received signals.